Liquids are commonly drawn from wells using lift pumps. For example, in the case of oil it is common to use a reciprocating pump having a plunger reciprocating in a pump barrel. An inlet check valve admits oil into the barrel below the plunger on the upstroke and a second check valve allows the liquid to flow past the plunger into a tubing string leading to the surface. The plunger is pulled up on the upstroke by a tension element, which is usually a rod, but may be some other type of element, sometimes the tubing string itself. This lifts the column of liquid in the tubing string towards the surface, and draws additional liquid into the barrel through the inlet check valve. On the down stroke of the pump, the plunger descends by gravity and oil that is captured in the barrel passes through the second check valve to be lifted by the plunger in the next upstroke.
A major limitation on the delivery flow rate of a pumping system of this sort is the speed at which the plunger and tension element will descend on the down stroke. Down strokes of the tension element can be resisted by viscous drag forces of a significant magnitude. This resistance cannot be overcome by large axial compression forces on the tension element without unacceptable bending. Consequently, the plungers and rods descend slowly, against the resistance of the liquid to be pumped. This resistance can be quite high. With heavy oils, it becomes the limiting factor on the rate of delivery from any given well.
The present invention therefore aims at the provision of methods and apparatus for mitigating this problem.